In the production of a filler layer component, and specifically of a filler layer capacitor, one normally starts with a sintered ceramic base element which is composed of dense and porous zones, e.g. alternating layers, and the porous zones of which are impregnated in a separate process step with a liquid metal or metal alloy. The ceramic elements required for this process are known in the art and may be produced according to German Patent DE-PS No. 24 62 007 or U.S. Pat. Nos. 3,879,645 and 4,353,957, which are incorporated herein by reference.
In the interest of a low process temperature, metals of low melting point, as e.g. lead and/or lead alloys, are preferred as the impregnating material. Due to the generally unfavorable wetting tensions between ceramic and impregnating material on the one hand, and the generally low height dimensions of the porous zones to be impregnated (e.g. 2 to 10 microns), on the other, pressure must be applied in the impregnating process. Such manufacturing methods are well known in the art.
In using the filler layer component as a capacitor, contacts of a suitable metallic material are applied to the ceramic body at the points of transfer of the impregnating metal. As a rule, silver based materials are chosen for this. If prior to the impregnation process these contacts are fired onto the ceramic surface, equally passable porosities must be provided which will prevent bleeding of the liquid impregnating material from the porous zones of the ceramic body when it is extracted from the impregnating bath. The reason for this is the tendency of conventional impregnating materials to form a partial alloy with the silver based material of the contacts, or at the least, result in a good wetting between the two materials. Alloy formation after a preceding dissolution reaction is desirable, however, to ensure a satisfactory passage of electric current, i.e. to ensure good bonding between the metallic filler in the porous zones and the contacts.
The dissolution reaction, however, has the disadvantage that during the pressure impregnation process the contacts are caused by the impregnating bath to undergo incipient dissolution to an impermissibly high degree and in the worst case are even almost detached.
In Great Britain Patent GB-A-No. 2104291, an aluminum containing glass frit is fired onto the ceramic body for the filler layer component before impregnation. While such a layer is resistant to the impregnating bath, it cannot be soldered, making it necessary to apply additional solderable layers on the first layer in subsequent operations.
In U.S. pat. No. 4,450,502 it is further proposed to apply on the ceramic body comprising silver or silver alloy and a glass frit, an external termination which is fired in and then coated with silver sulfide by a heat treatment at 350.degree. C. to 400.degree. C. with sulfur vapor or hydrogen sulfide vapor. Detachment of the silver layer from the ceramic body would thereby be avoided.
Thus, in the latter described process, additional steps must be carried out, and the substances used are highly toxic. During production of the terminations nascent hydrogen forms, which reduces the ceramic materials and may result in a worsening of the dielectric properties of the ceramic body.
It is, therefore, an object of the present invention to provide a structure and composition for the contacts so as to provide a good electrical bond to the metallic filler as well as ensuring resistance to detachment during the impregnating bath, and particularly in a lead or lead alloy bath.